The invention relates to power converter systems and more specifically to a system and method for controlling common mode voltages generated in power converter systems.
Typically, a three phase power converter system includes a power source that generates variable frequency AC input power, a rectifier, an inverter and a load. The rectifier operates to convert the AC input power to corresponding DC power. The inverter operates to convert the DC power to AC output power of a desired frequency. The AC output power is used to drive a load such as a motor.
The rectifier and inverter typically include switching devices such as solid state switches (e.g., insulated gate bipolar transistors) and diodes. In rectifiers or inverters that employ solid state switches, the switching operation is performed in a controlled fashion to obtain a desired output power. The switches may be controlled using a control system, which typically includes a control circuit based on a processor, such as a microprocessor, and drive circuitry for driving the gates of the switches.
Common mode voltages are typically generated due to the switching operation of the inverter. Under various operating conditions, these common mode voltages can become fairly high in amplitude and can potentially reach levels that are beyond the insulation ratings of the motor windings (or other load rating). Consequently, running of the load in a manner resulting in such common mode voltages can risk damaging the load or decrease its life expectancy.
A variety of modified power converter designs have been developed to reduce or filter the common mode voltages arising out of the switching of the inverter in such power converter systems. However, the modified power converter designs either fail to reduce common mode voltage to satisfactory levels or introduce other disadvantages. For example, some modified power converter designs attempt to reduce the levels of common mode voltages by controlling the pulsing on and off of the solid state switching devices within the power converters.
However, such modified power converters are more complicated to operate and control, achieve their results at a cost to the power converters' modulation indices and/or total harmonic distortion levels of voltage and current, and in any event fail to substantially reduce the common mode voltages.
Other modified power converter designs attempt to compensate for and nullify the common mode voltages by producing negative voltages through the use of additional switches, or through the use of isolation transformers. However, the use of additional switches can increase the complexity of controlling operation of the power converters, and can increase the cost of the power converters by increasing the number of circuit components.
Further, where transformers are employed, the transformers must be rated to handle the common mode voltage levels. Also, the use of such transformers increases the cost of the power converters and, due to the size of the transformers, can increase the bulkiness of the power converters.
Other techniques require modifications that reconfigure the power converter structure to render the mid point of the DC bus available to the user. Such a provision is not available in a standard power converter. Other techniques also require the neutral point of the load such as a motor to be accessible and connected to the power converter which is not available. In particular, this configuration is not applicable to motors with a Delta connected stator windings.
Therefore, there is a need to design a common mode filter to reduce the common mode voltages in a way that does not increase the complexity of the design, the cost, or the size of the power converters. At the same time, it is also desirable to design the common mode filter in a way such that it can be easily fitted with standard power converters.